Fine mesh screening

ABSTRACT

A screening system having a resiliently mounted frame with a screen extending thereacross. The frame is vibrated by a low frequency vibratory drive using eccentric weights. About the peripheral frame of the screen, a high frequency drive or drives is employed to vibrate the screen in the range of 20,000 Hz. The high frequency vibration is generated at the peripheral frame about the screen. The screen is responsive to the high frequency vibrations in a plate-like manner rather than as a membrane.

This application is a continuation of application Ser. No. 08/094,850filed Jul. 20, 1993, issued Mar. 21, 1995 as U.S. Pat. No. 5,398,816.

BACKGROUND OF THE INVENTION

The field of the present invention is fine mesh screening systemsincluding the use of high frequency vibration.

Traditional vibratory screening structures typically include a base, aframe resiliently mounted on the base with a screen or screens extendingacross the frame. A low frequency vibratory drive in the speed range of8 Hz to 30 Hz is mounted to the frame with eccentric weights. Specificvibratory motions are established in the frame by the low frequencyvibratory drive, generating screen accelerations up to the 7 g range.

The foregoing devices have been used for screening fine materials andpowders. Stainless steel woven mesh screens having interstices in the 30to 150 micron range are used for such commercial processing. Thesedelicate, woven meshes are typically thin and comparatively limp. Themesh is usually stretched tightly and attached to a screen frame. Thevibration of such devices typically enhances gravity separation ofparticles presented to the screen. Where fine particles are to bescreened, the vibration also has a deleterious effect in that the fineparticles become suspended above a boundary layer over the vibratingscreen.

Fine mesh screens can be reasonably fragile under many if not mostapplications. Backing screens and perforated plates have been used tohelp support such screens. Such supported screens may be bonded orunbonded. One type of area bonding is diffusion bonding. In someinstances, a fine screen cloth, a coarse screen cloth and a perforatedplate have been used. The separate layers are bonded by heat andpressure into a unitary structure. A frame is typically used about theperiphery of the composite screen structure for support, mounting andscreen tension.

In an effort to overcome the deficiency of low frequency vibration, highfrequency vibration has been employed. Ultrasonic vibrators have beenmounted to separator frames with a direct mechanical attachment to thescreens at the centers thereof. Alternatively, ultrasonic drives havebeen bonded directly to the screen. Fine mesh, tensioned screens tend tosuffer fatigue failure at the boundary layer with the mechanicalcoupling to the ultrasonic drive and to dissipate energy within a fewinches of that coupling. Such devices also use low frequency vibratorydrives, principally as a means for conveying material across the screenor screens.

SUMMARY OF THE INVENTION

The present invention is directed to screening systems using screensstiff as to bending with a high frequency drive mounted to drive througha frame structure rather than through the screen cloth directly.Substantial energy can be transmitted across a wide area of screen clothwhile fatigue failure of the screen at the boundary with the ultrasonicattachment is eliminated.

In a first, separate aspect of the present invention, a screen stiffenedto have predominantly plate-like properties as measured at 20,000 Hz isemployed for high frequency screening. Plate-like properties includemotion behavior influenced more by bending than by tension. Longwavelength high frequency motion is promoted by such screen stiffness.High damping of short wavelength energy transmitted in thin screensacting as tension membranes is significantly reduced. Thus, substantialhigh frequency energy distribution can be effected through theemployment of a stiff screen.

In a second, separate aspect of the present invention, a high frequencydrive or drives of a vibratory screening system having a resilientlymounted frame are provided at a frame about the screen, either throughthe resiliently mounted frame of the screening device or through a rigidscreen frame. Direct attachment of the high frequency drive to thescreen with attendant fatigue failure is avoided.

In yet a further, separate aspect of the present invention, highfrequency vibration is provided to a screen frame, either directly orthrough a resiliently mounted separator frame of a vibratory screeningsystem, with a stiff screen which behaves plate-like at highfrequencies.

Thus, improved fine particle screening is achieved. Other objects andadvantages will appear hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevation of a vibratory separator employing high frequencydrives.

FIG. 2 is a cross-sectional detail of a separator illustrating a firstmounting of a high frequency drive.

FIG. 3 is a cross-sectional detail illustrating a second mounting of ahigh frequency drive.

FIG. 4 is a cross-sectional detail of a second embodiment showing afirst transducer mounting and an isolated frame.

FIG. 5 is a cross-sectional detail of a second embodiment showing asecond transducer mounting and an isolated frame.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning in detail to the drawings, FIG. 1 illustrates a vibratory screenseparator. The separator includes a base 10 resiliently mounting aseparator frame 12 by means of springs 14. The separator frame 12 isillustrated here to be cylindrical, open at the top to receive materialinput and having discharge ports 16 and 18 for the screened material andthe oversize material, respectively. A low frequency vibratory drive 20is rigidly fixed to the separator frame 12. The drive 20 includes upperand lower eccentric weights 22 and 24 to generate a vibratory motionwhen rotatably driven.

A screen 26 extends across the separator frame 12 such that materialinput above the screen 26 must either pass through the screen or throughthe oversize discharge port 18. The screen 26 includes a screeningelement 28 stretched in tension uniformly by a screen frame 30. In thepreferred embodiments, the screening element 28 is a composite of a finemesh screen of a desired size with a stiffer porous sheet, mostconveniently a perforated plate. Diffusion bonding is employed acrossthe full area of the screening element 28. Such bonded screeningelements are commercially available.

To properly bond fine mesh screen cloth to a perforated plate withdiffusion bonding, it is preferable to include a coarse mesh screencloth therebetween. A fine mesh screen 32, a coarse mesh screen 34 and aperforated plate 36 are shown. These are supplied as a diffusion bondedlaminate which is tensioned and bonded to a screen frame 30 to form thescreen structure 26. The fine mesh screen cloth may be dictated by therequirements of the materials being screened. 200 mesh and 325 meshscreen cloth is common. The backing perforated plate is preferably 80%open and is from 1/16" to 3/16" thick. Design choice may dictate thinneror thicker plates depending on separator size, weight of material,degree of low frequency vibrations and the like.

In vibration theory, one might attempt to analyze vibration of a thinplanar object as either a membrane where the tension forces in the planeof the object dominate or as a plate where resistance to bending, orstiffness, of the planar object dominates. The propagation constant orwavenumber for membrane vibration is: ##EQU1## where λ=wavelength

ω=frequency radians per second

C=velocity of waves

ρ=mass density

T=static tension

The wavenumber for a plate is: ##EQU2## where D=plate bending stiffness=##EQU3## E=Young's Modulus h=thickness of the plate

υ=Poisson's ratio.

To determine appropriate screen characteristics, the relative effects ofstiffness and tension may be determined by calculating the ratio of thetwo wavenumbers: ##EQU4## When this ratio is much less than unity,tension dominates and membrane theory applies. When the ratio is muchgreater than unity, stiffness dominates; therefore, the screen behavesin a plate-like manner. Using this formula, screens exhibiting a ratioin excess of unity tend to act plate-like while screens exhibiting aratio of less than unity tend to act membrane like. In estimating platebending stiffness, the standard Poisson's ratio for stainless steel maybe reduced to 0.2 to account for the relief provided by the holes in thescreen. Calculations of unsupported and supported screens at a vibrationfrequency of 20,000 Hz establish the following values:

    ______________________________________                                                   Mass      Plate       Wavenumber                                              Density   Stiffness,  Ratio                                        Screen     ρ, g/m.sup.3                                                                        D, newton-mm                                                                              (Eq. 4)                                      ______________________________________                                        200 mesh   298       0.38        0.67                                         325 mesh   209       0.19        0.51                                         200 mesh bonded                                                                          696       13.3        2.02                                         325 mesh bonded                                                                          611       11.6        1.88                                         ______________________________________                                    

In running a test unit, using an 18 inch diameter 200 mesh (74 micron)diffusion bonded screen having the properties as indicated in the table,excitation frequency was set at approximately 20,000 Hz. One hundredwatts of power was employed which appears to have been more thansufficient. Increasing the wattage did not appear to significantlyincrease screening efficiency. Screening efficiency may in fact increasewith decreased wattage and adjustment of the vibration pattern.Effective high frequency vibration is understood to be in the range offrom about 10,000 Hz to 50,000 Hz. The plate-like behavior of the screenhas suggested that less tension may be necessary as well.

Empirical results suggest close to double the screening efficiency ofthe same system without high frequency vibration. One or more highfrequency generators 38 are associated with the peripheral frame.Clearly, empirical testing as to the number of generators 38, theirplacement and orientation for the characteristics of each material beingprocessed is appropriately conducted. An increased number of generators38 provides greater flexibility and uniformity of high frequency energycoverage. However, increasing the number of generators 38 increases costand complexity. Several types of such generators are available. It ispresently believed that magnetostrictive ultrasonic transducers withinternal feedback control are preferred as they are more rugged for shopuse and have a wider frequency band.

As illustrated in FIGS. 2 through 5, two types of screen mountings areemployed. In FIGS. 2 and 3, the separator frame 12 directly supports thescreen 26. The high frequency generator 38 is shown to be mountedrigidly to the separator frame 12 by a fixed bracket 40. The action ofthe high frequency generator or generators 38 through the fixed bracketor brackets 40 on the separator frame 12 is transferred from thatperipheral frame to the screen 26. The separator frame 12 in itsentirety is also subject to be vibrated in this arrangement.

An alternative arrangement is illustrated in FIGS. 4 and 5. In thesefigures, resilient elements or gaskets 42 and 44 are positioned on topand bottom of the screen frame 30 to isolate the screen frame from thesurrounding separator frame 12. The resilient elements would act toisolate the separator frame 12 but would also act to damp some of thepower generated by the generator(s) 38. Again, the vibration isintroduced by means of a peripheral frame to the screen 26.

Low frequency vibration is employed at levels allowing the separatorframe 12 and screen 26 to vibrate as a rigid body, even in theembodiment where the resilient elements 42 and 44 are employed. Smallerpower and lighter weights may be employed for the low frequencyvibratory drive as compared with conventional low frequency separatorssince this drive is now relegated to transportation of material acrossthe screen. Low frequency vibrations effective for conveying materialare typically considered most effective in the 3 Hz to 30 Hz range.Energy typically effective for conveyance of fine material on a screenmay be measured in screen acceleration in the 1/2 g to 4 g range at 20Hz.

Accordingly, an improved fine mesh screening system is disclosed. Whileembodiments and applications of this invention have been shown anddescribed, it would be apparent to those skilled in the art that manymore modifications are possible without departing from the inventiveconcepts herein. The invention, therefore is not to be restricted exceptin the spirit of the appended claims.

What is claimed is:
 1. A screening system comprisinga resilientlymounted frame; a low frequency vibratory drive coupled to the frame; ascreen extending across the frame, the screen including a screen frame;a plurality of high frequency drives mounted rigidly to the periphery ofthe screen, to vibrate the screen above about 10,000 Hz, the highfrequency drives being rigidly mounted to the screen frame; a resilientmounting between the screen and the screen frame; and brackets, each ofthe high frequency drives being rigidly fixed to one of the brackets,respectively, the brackets being rigidly fixed to the screen frame torigidly mount the high frequency drives to the periphery of the screen.